diff --git a/plugins/kolab_auth/kolab_auth_ldap.php b/plugins/kolab_auth/kolab_auth_ldap.php
--- a/plugins/kolab_auth/kolab_auth_ldap.php
+++ b/plugins/kolab_auth/kolab_auth_ldap.php
@@ -37,6 +37,9 @@
 
         $this->conf = $p;
         $this->conf['kolab_auth_user_displayname'] = $rcmail->config->get('kolab_auth_user_displayname', '{name}');
+        $this->conf['kolab_domain_name_attribute'] = $rcmail->config->get('kolab_domain_name_attribute', 'associateddomain');
+        $this->conf['kolab_domain_base_dn'] = $rcmail->config->get('kolab_domain_base_dn', 'cn=kolab,cn=config');
+        $this->conf['debug_level'] = $rcmail->config->get('debug_level', 0);
 
         $this->fieldmap = $p['fieldmap'];
         $this->fieldmap['uid'] = 'uid';
@@ -103,6 +106,48 @@
     }
 
     /**
+     * Get the mail address of the user uniquely identified with the UID, checking all domains available
+     */
+    function get_mail_of_user_across_domains($user, $filter)
+    {
+        $count = 0;
+        $mail = '';
+
+        if ($result = parent::search($this->conf['kolab_domain_base_dn'], '', '', array($this->conf['kolab_domain_name_attribute']))) {
+            if ($result->count() > 0) {
+                foreach ($result->entries(true) as $dn => $attrs) {
+                    $domain = $attrs[$this->conf['kolab_domain_name_attribute']];
+                    if (is_array($domain)) {
+                        $dc = $this->domain_root_dn($domain[0]);
+                    } else {
+                        $dc = $this->domain_root_dn($domain);
+                    }
+
+                    // check if the user lives in this domain
+                    if ($result2 = parent::search('ou=people,'.$dc, $filter, '', array('mail'))) {
+                        $count += $result2->count();
+                        if ($result2->count() == 1) {
+                            $entries = $result2->entries(true);
+                            $entry = array_pop($entries);
+                            $mail = $entry['mail'];
+                        }
+                    }
+                }
+            }
+        }
+
+        if ($count == 1) {
+            if ($this->conf['debug_level'] > 0) {
+	            rcube::console("Authentication: use mail address $mail for user with UID $user");
+            }
+            return $mail;
+        } else if ($count > 0) {
+            rcube::write_log('errors', "Authentication: found multiple users with UID $user, therefore cancelling login");
+        }
+        return False;
+    }
+
+    /**
      * Fetches user data from LDAP addressbook
      */
     function get_user_record($user, $host)
@@ -124,6 +169,10 @@
                 $entry   = $this->field_mapping($dn, $entry);
 
                 return $entry;
+            } else {
+               if ($mail = $this->get_mail_of_user_across_domains($user, $filter)) {
+                    return $this->get_user_record($mail, $host);
+               }
             }
         }
     }